1. Field of the Invention
The present invention relates to a fingertip pulse wave sensor for detecting changes in the volume of a blood vessel in a fingertip held between a light-emitting element and a light-receiving element, in which the changes are caused by pulsation and detected as electrical signals produced in response to changes in the intensity of the light transmitted through the fingertip.
2. Description of the Related Art
A conventional fingertip pulse wave sensor of the above-described type includes a pad with either a light-receiving element or a light-emitting element buried therein which makes contact with a fingernail, and a pad which has the other of the light-receiving and light-emitting elements buried therein and which makes contact with a fingertip cushion. These pads are disposed in opposed relationship with each other. The conventional fingertip pulse wave detector also includes a plate spring having a U-shaped form and is mounted on the rear surface of either of the pads so as to hold the fingertip between the two pads and under pressure.
With this arrangement, although the fingertip can be elastically held in place in spite of the variations naturally occurring in the shape of the fingertips of different individuals or any irregularities in the top dead point of the plate spring employed, these variations or irregularities serve to cause variations in the amount of pressure applied to the finger to a considerable extent. For example, in the case of a spring designed to provide a force that will press a fingertip with a deflection of 3 mm, if a deflection of 6 mm is caused due to differences in the shape of different individuals' fingertips or irregularities in the top dead point of the spring employed, it is possible that the pressure applied to the fingertip will be completely different. Excessive pressure exerted on a portion to be measured, increases the amount of blood in the venula of the fingertip, which is being returned to the associated vein. This has an effect on the waveform of a pulse, which is produced during arterial pulse wave detection at the fingertip.
This is undesirable in terms of the measurement precision in a plethysmograph which is designed to estimate the form of a waveform in its detected state. This greatly affects the results of measurements obtained using an "acceleration type" fingertip pulse wave detector which must be able to non-invasively monitor the state of a peripheral circle by converting the waveform detected to an acceleration curve. This conversion is achieved by differentiating twice the detected waveform with respect to the time. This conversion requirement, in effect, prevents such an "acceleration-type" fingertip pulse wave detector from being employable in clinical applications.